Ferromagnetic materials that are known to be usable in magneto-optical recording media include amorphous alloys of a rare earth element and transition metal system such as TbFeCo and GdCo, polycrystalline alloys such as MnBi, MnCuBi and PtCo, and cobalt ferrite and iron garnet. The polycrystalline materials have not yet been commercialized because the grains in the thin film form are not small enough to suppress noise in read signals and because of their low recording sensitivity. Therefore, most of the efforts currently made to commercialize magneto-optical recording media are based on the use of thin films of amorphous alloys of the rare earth/transition metal system which exhibit superior overall characteristics. However, these amorphous alloys do not have sufficiently large magneto-optical effects to provide reasonably high signal-to-noise ratios. The Kerr rotation angles, for example, are small. Therefore, attempts are being made to enhance their magneto-optical effects of these amorphous alloys by various means, including forming a two- or three-layered structure containing not only the amorphous alloy layer but also a dielectric layer having an appropriate refractive index, as described, for example, in Ohta et al., Reference Material for the 27th Annual Meeting of The Magnetics Society of Japan, p. 57; and Inoue et al., Summary of the Proceedings of the 8th Conference of The Magnetics Society of Japan, p. 141.
In recent years, scientists have engaged in active studies on the physical properties of thin films in which two different materials form alternating ultrathin layers. Some of these studies have considered the magneto-optical approaches, as described, for example, in Katayama et al., The Proceedings of International Symposium on Physics of Magnetic Materials, 1987, p. 283; and Togami et al., The Proceedings of International Symposium on Physics of Magnetic Materials, 1987, p. 275). However, no magneto-optical thin film has yet been proposed that attains useful results such as an improvement in magneto-optical effects.
As already mentioned, thin films of amorphous alloys of the rare earth/transition metal system which are presently considered to be the best for use as ferromagnetic thin films in magneto-optical recording media have small Kerr rotation angles. In order to solve this problem, the Kerr rotation angles (.theta.k) of these films are increased by laminating them to dielectric layers whose thickness is adjusted to satisfy a near non-reflection condition. However, the reflectance of such laminates is then reduced. In magneto-optical recording, the play back signal to noise ratio is generally proportional to .sqroot.R.theta.k (R:reflectance), so reduction in reflectance is not desirable. Because of this problem, the magneto-optical thin films available today fall short of attaining commercially satisfactory C/N (carrier to noise) ratio. Therefore, a strong need exists for a magneto-optical thin film having a value of .sqroot.R.theta.k higher than those current available.